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Full text of "The Cambridge Natural History"

OVA OF MAMMALS

and on the left being different. The lungs of mammals agree
with those of the lower reptiles in being freely suspended within
their coelomic cavity, and in not being, as In birds, crocodiles,
and the Varanidae among lizards, tied down to the dorsal surface
of that cavity by a sheet of peritoneum covering them.

The Gonads (Ovaries and Testes).—The ovary in the
Mammalia Is always paired; there is never a partial or com-
plete abortion of one gonad as in birds—except of course in
pathological cases. The ovaries are small, and lie in the

FIG. 44.—Part of a sagittal section, of an ovary of a child just born. W.», Blood-vessels ;
foil, strings and groups of cells derived from the germinal epithelium becoming
developed into follicles ; ff-epf germinal epithelium; in, ingrowing cord of cells
from the germinal epithelium ; pr.ov} primitive ova. (From Hertwig, after
Waldeyer.)
abdominal cavity behind the kidneys. In ttie immense majority
of the Mammalia the ova which are produced within the
ovaries are of minute size; those of even the colossal Rorqual
are, so far as we know, not markedly larger than the ova of a
Mouse. The smallness of size of these reproductive elements
implies necessarily an absence of much nutritive yolk; and as a
consequence the developing embryo, since it is not hatched in an
early stage as a free living larva, has to be nourished by the
mother, to whose tissues it is attached through the intermediary
of the placenta, a structure partly composed of foetal structures
derived from the embryo, a&4 partly of portions of the lining
of the uterus of the mother* The ova, of the